A movement disorder is a neurological disturbance that involves one or more muscles or muscle groups. Movement disorders affect a significant portion of the population, causing disability as well as distress. Movement disorders include Parkinson's disease, Huntington's chorea, progressive supranuclear palsy, Wilson's disease, Tourette's syndrome, epilepsy, tardive dyskinesia, and various chronic tremors, tics and dystonias. Different clinically observed movement disorders can be traced to the same or similar areas of the brain. For example, abnormalities of basal ganglia (a large cluster of cells deep in the hemispheres of the brain) are postulated as a causative factor in diverse movement disorders.
Parkinson's disease is a movement disorder of increasing occurrence in aging populations. Parkinson's disease is a common disabling disease of old age affecting about one percent of the population over the age of 60 in the United States. The incidence of Parkinson's disease increases with age and the cumulative lifetime risk of an individual developing the disease is about 1 in 40. Symptoms include pronounced tremor of the extremities, bradykinesia, rigidity and postural change. A perceived pathophysiological cause of Parkinson's disease is progressive destruction of dopamine producing cells in the basal ganglia which comprise the pars compartum of the substantia nigra, basal nuclei located in the brain stern. Loss of dopamineric neurons results in a relative excess of acetylcholine. Jellinger, K. A., Post Mortem Studies in Parkinson's Disease—Is It Possible to Detect Brain Areas For Specific Symptoms?, J Neural Transm 56 (Supp); 1-29:1999. Parkinson's disease is a progressive disorder which can begin with mild limb stiffness and infrequent tremors and progress over a period of ten or more years to frequent tremors and memory impairment, to uncontrollable tremors and dementia.
Tardive dyskinesia (TD) is a chronic disorder of the nervous system, characterized by involuntary, irregular rhythmic movements of the mouth, tongue, and facial muscles. The upper extremities also may be involved. These movements may be accompanied, to a variable extent, by other involuntary movements and movement disorders. These include rocking, writhing, or twisting movements of the trunk (tardive dystonia), forcible eye closure (tardive blepharospasm), an irresistible impulse to move continually (tardive akathisia), jerking movements of the neck (tardive spasmodic torticollis), and disrupted respiratory movements (respiratory dyskinesia). The vast majority of TD cases are caused by the prolonged use of antipsychotic drugs (neuroleptics). A relatively small number are caused by the use of other medications, such as metoclopramide, that, like neuroleptics, block dopamine receptors. TD often manifests or worsens in severity after neuroleptic drug therapy is discontinued. Resumption of neuroleptic therapy will temporarily suppress the involuntary movements, but may aggravate them in the long run.
Tardive dyskinesia affects approximately 15-20% of patients treated with neuroleptic drugs (Khot et al., Neuroleptics and Classic Tardive Dyskinesia, in Lang A E, Weiner W J (eds.): Drug Induced Movement Disorders, Futura Publishing Co., 1992, pp 121-166). Therefore, the condition affects hundreds of thousands of people in the United States alone. The cumulative incidence of TD is substantially higher in women, in older people, and in those being treated with neuroleptics for conditions other than schizophrenia, such as bipolar disorder (manic-depressive illness) (see, e.g., Hayashi et al., Clin. Neuropharmacol, 19:390, 1996; Jeste et al., Arch. Gen. Psychiatry, 52:756, 1995). Unlike the acute motor side effects of neuroleptic drugs, TD does not respond in general to antiparkinson drugs (Decker et al., New Eng. J Med., Oct. 7, p. 861, 1971).
Focal dystonias are a class of related movement disorders involving the intermittent sustained contraction of a group of muscles. The prevalence of focal dystonias in one US county was estimated as 287 per million (Monroe County Study); this suggests that at least 70,000 people are affected in the US alone. The spasms of focal dystonia can last many seconds at a time, causing major disruption of the function of the affected area. Some of the focal dystonias are precipitated by repetitive movements; writer's cramp is the best known example. Focal dystonia can involve the face (e.g., blepharospasm, mandibular dystonia), the neck (torticollis), the limbs (e.g., writer's cramp), or the trunk. Dystonia can occur spontaneously or can be precipitated by exposure to neuroleptic drugs and other dopamine receptor blockers (tardive dystonia). No systemic drug therapy is generally effective, but some drugs give partial relief to some patients. Those most often prescribed are anticholinergics, baclofen, benzodiazepines, and dopamine agonists and antagonists. The most consistently effective treatment is the injection of botulinum toxin into affected muscles.
The various focal dystonias tend to respond to the same drugs (Chen, Clin. Orthop, June, 102-6, 1998; Esper et al; Term. Med, January, 90:18-20, 1997; De Mattos et al., Arq. Neuropsychiatry, March 54:30-6, 1996) This suggests that a new treatment helpful for one focal dystonia would be likely to be helpful for another. Furthermore, the common symptoms, signs, and responses to medication of spontaneous (idiopathic) dystonia and neuroleptic-induced dystonia suggest that an effective treatment for a drug-induced focal dystonia will be effective for the same dystonia occurring spontaneously.
A tic is an abrupt repetitive movement, gesture, or utterance that often mimics a normal type of behavior. Motor tics include movements such as eye blinking, head jerks or shoulder shrugs, but can vary to more complex purposive appearing behaviors such as facial expressions of emotion or meaningful gestures of the arms and head. In extreme cases, the movement can be obscene (copropraxia) or self injurious. Phonic or vocal tics range from throat clearing sounds to complex vocalizations and speech, sometimes with coprolalia (obscene speech) (Leckman et al., supra). Tics are irregular in time, though consistent regarding the muscle groups involved. Characteristically, they can be suppressed for a short time by voluntary effort.
Tics are estimated to affect 1% to 13% of boys and 1% to 11% of girls, the male-female ratio being less than 2 to 1. Approximately 5% of children between the ages of 7 and 11 years are affected with tic behavior (Leckman et al., Neuropsychiatry of the Bas. Gang, December, 20(4): 839-861, 1997). The estimated prevalence of multiple tics with vocalization, i.e. Tourette's syndrome, varies among different reports, ranging from 5 per 10,000 to 5 per 1,000. Tourette's syndrome is 3-4 times more common in boys than girls and 10 times more common in children and adolescents than in adults (Leckman et al., supra; Esper et al, Term. Med. 90:18-20, 1997).
Gilles de la Tourette syndrome (TS) is the most severe tic disorder. Patients with TS have multiple tics, including at least one vocal (phonic) tic. TS becomes apparent in early childhood with the presentation of simple motor tics, for example, eye blinking or head jerks. Initially, tics may come and go, but in time tics become persistent and severe, and begin to have adverse effects on the child and the child's family. Phonic tics manifest, on average, 1 to 2 years after the onset of motor tics. By the age of 10, most children have developed an awareness of the premonitory urges that frequently precede a tic. Such premonitions may enable the individual to voluntary suppress the tic, yet premonition unfortunately adds to the discomfort associated with having the disorder. By late adolescence/early adulthood, tic disorders can improve significantly in certain individuals. However, adults who continue to suffer from tics often have particularly severe and debilitating symptoms. (Leckman et al., supra).
Although the present day pharmacopeia offers a variety of agents to treat movement disorders, none of these agents can prevent or cure these conditions. Furthermore, the most effective treatments are often associated with intolerable side effects. There remains a clear-cut need for new treatments for movement disorders that have greater efficacy and fewer side effects than those currently available.